(a) Field of the invention
The present invention relates to a continuous alcohol manufacturing process using yeast, in particular an alcohol manufacturing process which is designed to effect continuous alcohol fermentation by making three kinds of yeasts having an alcohol producing activity respectively (three kinds of alcohol fermentation yeasts) be present individually in series and introducing a substrate solution in each of them.
(b) Description of the prior art
Alcohol (ethyl alcohol) is widely used for not only drinking but also various industrial purposes, and is manufactured by the fermentation method and the synthesis method. In the manufacture of alcohol on an industrial scale according to the fermentation method, where molasses is used as the starting material, part of said molasses is first diluted with hot water to such an extent that its total sugar content becomes about 15 wt./V%, then a nutrient is added thereto for pH adjusting purposes, and the same is sterilized. After completion of said sterilization, the same is cooled to about 30.degree. C., and then is inoculated with yeast fungus and cultured for about 2 days to prepare a seed culture (seed yeast). On the other hand, the remainder of said molasses is diluted with hot water to such an extent that its total sugar content becomes about 25 wt./V%, then the aforesaid seed yeast is added thereto, and thus fermentation is conducted at about 30.degree. C. The fermentation is completed in about 4 days and a broth containing about 13 vol% alcohol is prepared. Successively, the yeast is removed from the obtained alcohol broth, and the latter is distilled to obtain alcohol (ethyl alcohol).
However, the fact is that the aforesaid manufacture of alcohol according to the fermentation method is conducted in a batch culture, and that said batch culture includes drawbacks or disadvantages to be solved, for instance, (1) since the yeast concentration is in a low range such as 3-4 g/l, a large capacity fermentor is needed in order to produce alcohol in a desired amount, (2) since the reaction speed is slow, the fermentation time is prolonged and consequently the productivity is deteriorated, and (3) since the alcohol and the other by-products strongly retard the reaction speed, the concentration of product alcohol must naturally be reduced from the viewpoint of manufacturing alcohol economically, and the like.
In order to solve these problems and profitably carry out the manufacture of alcohol according to the fermentation method, as a matter of course there have been proposed various procedures, for instance, to conduct the manufacture of alcohol continuously while dispersing yeast in a substrate solution, to prepare an immobilized yeast by immobilizing a living yeast on a carrier and utilize this (immobilized yeast) in a living or growing state and so forth.
The alcohol fermentation method using this immobilized yeast has been proposed very recently. For instance, a continuous alcohol manufacturing method is disclosed which comprises supplying a glucose-containing nutritional medium to an immobilized yeast obtained by immobilizing a concentrated yeast (Saccharomyces carbergensis) with carrageenan or a concentrated yeast (Saccharomyces cerevisiae) with polyacrylamide [which see "Kagaku Kozyo" (Chemical Factories) vol. 23, No. 3, pp.26-30 (published by Nikkan Kogyo Shinbun Sha)].
The continuous culture to be practiced without immobilizing yeast as above mentioned (namely, while dispersing yeast in a substrate solution) and the continuous culture using immobilized yeast may be expected to be more profitable than the conventional batch culture in that alcohol can be manufactured in a relatively short time and consequently the fermentor can achieve a high volumetric efficiency (alcohol productivity of fermentor per unit volume and unit time). On the other hand, however, there is no denying the fact that when the conventional yeast that has generally been used in alcohol fermentation is employed, even these continuous cultures must also reduce the concentration of the fermentation product alcohol as in the case of batch culture because said yeast is subject to retardments caused by the osmotic pressure of a substrate (sugar) and the product (alcohol).
In this connection, it is to be noted that the conventional yeast that has normally been used in alcohol fermentation can not endure a sugar concentration up to about 25 wt./V% at most, although this concentration is variable depending on the kind of yeast. When said concentration is in excess of this limit, the osmotic pressure of the substrate is so elevated that the activity of the yeast is reduced or restricted to an extreme degree. Further, in view of the fact that yeast loses its activity owing to the retarding action of the alcohol when the alcohol concentration is high, there is no possibility of expecting the manufacture of a broth having a high alcohol concentration. To cope with this, it is sometimes adopted to supply a batch of substrate solution (sugar solution) even in the case of batch system fermentation or to take a long period of time for fermentation, although variable depending on the kind of yeast, when there is a necessity of obtaining a broth of high alcohol concentration (alcohol concentration of about 18 vol.%), but these procedures are unprofitable.
It is found that the object of manufacturing alcohol economically as well as effectively can be achieved by obtaining a broth having an alcohol concentration upto about 13 vol./%. Accordingly, the fact is that the broth having the aforesaid alcohol concentration is now being manufactured.